1. Field of the Invention
This invention relates to improvements in buffer circuits, and more particularly to improvements in open loop type emitter follower and operational, or differential, amplifier types of buffer circuits.
2. Description of the Prior Art
Buffer amplifiers have wide applications, for example, to isolate signal paths, to insure proper signal levels from one stage to another, and so on. One widely used buffer circuit 5 is shown in FIG. 1. The circuit 5 of FIG. 1 is a so called "diamond configuration" buffer circuit, and includes two emitter follower input stages 10 and 11 and one output amplifier stage 12. The first emitter follower emitter stage 10 includes an NPN transistor 15 and a current source 16 in series with its emitter. Likewise, the second emitter follower circuit 11 includes a PNP transistor 18 with a current source 19 in series with its emitter. The output amplifier stage 12 includes an NPN transistor 21 and a PNP transistor 22, connected with their emitter-collector paths in series, as shown. The input signal to the circuit, denoted V.sub.IN is connected to the bases of the NPN transistor 15 and the PNP transistor 18, and the output of the circuit, denoted V.sub.OUT. is derived at an output node connection between the emitters of the NPN transistor 21 and the PNP transistor 22.
It can be seen that the buffer circuit 5 of FIG. 1 is an "open loop" amplifier, that is, it has its input signal directed to the output node with little delay or phase shift, with essentially no signal feedback. The circuit 5 is therefore relatively fast, has wide bandwidth performance, and has a good slew rate. Such circuits, however, nevertheless generally suffer from offset problems, and, since these circuits generally have very low output impedances, their output devices are usually required to be biased at very high currents.
Another type of buffer which is widely used is the "operational amplifier" or "differential amplifier" type buffer circuit 25, as shown in FIG. 2. As shown, the circuit 25 includes an operational amplifier 26 having a feedback path 27 connected between the output, V.sub.OUT and the negative, or inverting, input terminals. The input signal, V.sub.IN is connected to the positive, or non-inverting, input terminal of the operational amplifier 26.
In comparison with the "diamond configuration" buffer of the type shown in FIG. 1, the operational amplifier buffer of the type illustrated in FIG. 2 has excellent precision with low offset and low output impedance. Operational amplifier type buffers rely on the high gain of the operational amplifier to produce a good replica of V.sub.IN at their outputs. The drawback is that the operational amplifier must have usable gain over the entire desired frequency range of the buffer. Since, in practice, operational amplifier buffers often have a buffer similar to the above described open loop buffer as its output in a two stage arrangement with a feedback loop closed around the operational amplifier, the operational amplifier will necessarily be slower than the open loop type buffer.